Since March 2008, when San Francisco first put its map a click away from interested citizens, solar photovoltaic (PV) installations in the city have tripled, from 700 to 2,100 this year. "[It] is the main solar outreach tool used in San Francisco," said Danielle Murray, the city's renewable energy program manager.

That may be so, but many observers say the City by the Bay's rooftop solar boom has been largely driven by California's tiered rate system, in which electricity bills rise as people use more power. And, according to Murray, the solar map has averaged a less-than-arresting 70 hits a day since debuting — though she insists this figure belies the project's impact.

Seventeen other U.S. cities and the German city of Osnabrük have published solar maps on the Web since San Francisco's site went live, enabling home and building owners to assess the potential of their roofs to generate clean electricity.

The map combines aerial images with calculators and other features to provide owners with facts and figures needed when considering whether to purchase a PV system, such as the pitch of the roof and the amount of shade cast by neighboring buildings.